In the field of structures design, there is a constant search in the art for structures and materials which exhibit the properties of unlimited life and/or are structurally redundant for applications where fail-safe operation is desirable. For example, in the aerospace industry, it is essential that certain flight-safety-critical mechanical systems such as the main rotor hub assembly of rotorcraft operate continuously even in the presence of a structural flaw or failure. Accordingly, it is common practice for designers of such mechanical systems to employ structural members having the requisite mass and/or material composition for reacting the full spectrum of imposed loads or, alternatively, utilize multi-element construction for providing redundant load paths in the event of a single element failure.
Structural members of metallic composition offer ease of manufacturing by means of machining, casting or forging, however, such metallic structures typically exhibit poor flaw tolerance. As used herein, flaw tolerance is the ability of a structure to resist fatigue crack propagation, and, more importantly, to resist such propagation to the point of ultimate failure. To compensate for this material characteristic, the designer must ensure that adequate structure, i.e., material mass, is present to maintain normal and shear stresses at a level which prevents the formation or rapid growth of cracks in the structural member. It will be appreciated that design methodologies which increase the mass of the structure to effect fail-safe operation are structurally inefficient. Moreover, in a weight critical aircraft application, such structural inefficiency adversely impacts the fuel and flight performance of the aircraft.
Advances in material composition, such as those in the area of fiber-reinforced resin matrix materials, have provided more acceptable solutions to the requirement for fail-safety. Firstly, the reinforcing fibers therein inherently provide multiple load paths inasmuch as the structural fibers may be viewed as individual elements which, depending upon the fiber loading or content, are capable of redundantly reacting the imposed loads. Secondly, the failure mode of such composite materials is characterized by a delamination or matrix failure rather than a failure across the structural fibers. That is, cracks propagate in the binding matrix and do not significantly alter the structural integrity of the composite structure. Lastly, such composite structures offer superior strength to weight properties, hence are particularly advantageous for aircraft applications. Hibyan et al U.S. Pat. No. 4,585,393 discloses a light-weight, damage tolerant, yoke assembly for helicopter rotor hubs wherein a combination of unidirectionally oriented composite fibers, e.g., graphite and fiberglass, provides the desired fail-safety.
While composite materials offer these and other structural advantages, the complex geometry of many structural members, i.e., I-Beams, T-Beams and X-beams, is, oftentimes, fiscally unsuitable for composite manufacture, and, in particular, automated composite manufacture. Yao et al U.S. Pat. No. 4,650,401 describes a composite cruciform having a generally X-shaped cross-section wherein the cruciform functions to structurally interconnect a helicopter rotor blade assembly to a central torque driving hub member. It will be appreciated that manufacturing difficulties arise when attempting to arrange the fibers in the proper orientation to accommodate the various load paths through the rib sections of the cruciform. Accordingly, resort is made to laborious hand lay-up of composite material fibers to achieve the desired fiber orientation. Alternatively, simple rectangular shaped cross-sections such as employed in Hibyan et al, are employed to facilitate manufacturing while maintaining the desired fiber orientation.
While multi-element construction can simplify manufacturing by utilizing simple cross-sectional shapes, in combination to achieve the desired structural redundancy, such construction often requires increased spatial requirements and intricate assembly. McCafferty U.S. Pat. No. 4,264,277, describes an arrangement for redundantly mounting a helicopter rotor blade assembly to a hub member wherein a secondary spindle or tie bar is disposed internally of a primary arm for reacting rotor blade loads in the event of a primary arm failure. While the mounting arrangement disclosed therein provides separate load paths, it will be appreciated that such arrangement requires a large design envelop and involves intricate assembly.
A need therefore exists for providing a structural member which is structurally efficient, non-complex, and facilitates manufacture while having the desired flaw tolerance for fail-safe operation.